Source(s)

Chandra A, Martinez GM, Mosher WD, Abma JC, Jones J. Fertility, family planning, and reproductive health of U.S. women: Data from the 2002 National Survey of Family Growth. National Center for Health Statistics. Vital Health Stat 23(25). 2005.

Definitions

Includes all who have had sex at least once in the last 12 months.

Women were asked 'Have you ever used rhythm or safe period by calendar to prevent pregnancy?'

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Odds Statement provided by Book of Odds Inc., http://www.bookofodds.com/content/view/full/520027 (Accessed: )

Rounding Rules

Odds more likely than 1 in 10, such as 1 in 3.42, are rounded to three significant digits. Odds less likely than 1 in 10, such as 1 in 420.4, are rounded to four significant digits. Trailing zeros after the decimal point are dropped for readability, even though they may still be significant.

Odds you'll see: 1 in 4.26; 14.5; 143; 5,230; 433,200

Odds you won't see: 1 in 2.412; 63.042; 425,242c